United States, Russia, China and other nations are close to fielding hypersonic missiles that would be fired from jet fighters. Hypersonic missiles are closest because they are unmanned and do not need safety like manned vehicles. The hypersonic missiles are being carried to a speed where their hypersonic engines will work instead of combining multiple engines that are needed at different speeds like a hypersonic plane that takes off from the ground would need.

It is believed China's AVIC is developing a new high-altitude, hypersonic unmanned aerial vehicle requiring a carrier aircraft such as an H-6 twin-engine bomber. The drone will be launched once it is carried to a high enough altitude and will climb by itself to cruising altitude before traveling at hypersonic speed, KKTT said, adding that the drone will be able to return to base itself. China's drone would be a tiny incremental step beyond a hypersonic missile. It would be a hypersonic smart missile that could land.

The D-21 had a top speed of Mach 3.3-3.5, a service ceiling of 29,000 meters and a range of 5,550 kilometers. The Chinese UAV has a length of 12.8 meters, a wingspan of 5.8m and a launch weight of five tons. China is currently developing a Turbo Rocket Combined Cycle (TRCC) propulsion system that would enable top speeds between Mach 4.5-5. However, as the TRCC engine is still under development, it is more likely that the new drone is using a less advanced Rocket-Based Combined-Cycle (RBCC) engine.

The D-21 mounted on the back of the M-21. Note the intake cover on the drone, which was used on early flights.

The US Rocketdyne Trijet solution has an ejector ramjet. An Ejector ramjet is a ring of small rocket engines in the duct. The rockets ran fuel-rich and their high-velocity exhaust drew outside air through the inlet to burn the extra fuel. The rockets would burn hydrogen and oxygen, with the oxygen flow diminishing as the vehicle accelerated and shutting off at about Mach 2.5. Tests at Van Nuys showed that the rockets could propel the aircraft from standstill to high supersonic speed.

Again the new Chinese hypersonic drone could be simpler with a small regular rocket stage to boost it mach 4 before the air breathing stage kicked in.

An air breathing hypersonic engine would not need to carry its oxygen like regular rockets. This would save the weight of fuel. Hypersonic vehicles can thus be either smaller to do the same work as a rocket or they can carry more cargo. Hypersonic vehicles can also be made to maneuver more than a rocket.